1. Field of the Invention
This invention relates to heterodyne interferometers. Particularly, this invention relates to heterodyne interferometers and their application to measuring wavefronts, such as in characterizing the optical aberrations of adaptive deformable mirrors or the wavefront piston induced by a vibrating mirror.
2. Description of the Related Art
In optical wireless communications there is a need to measure the distortions which may affect transmitted communication signals. For example, a signal transmitted between a satellite and a ground station may undergo distortion as consequence of being transmitted through the atmosphere. Measurements of a signal wavefront, such as an optical signal transmitted through the atmosphere, may be used to factor out the distortions that may occur in the transmission of a communications signal. Great pointing precision and overall communication efficiency can be achieved with more accurate and efficient measurements of such distortions.
There are several known non-heterodyne interferometry techniques for measuring optical characteristics or optical wavefronts. These typically include some form of image processing to identify interference fringes. In addition, there are slope detecting sensors, such as the Shack-Hartmann wavefront sensor or the shearing interferometer, for measuring the local tip and tilt of an optical element of a wavefront.
However, such non-heterodyne techniques typically have relatively slow acquisition and processing times which limit their usefulness in high bandwidth systems. The slope detecting techniques require substantial matrix computations to reconstruct optical path measurements, for example. This intensive processing can also increase the necessary processing time for such techniques.
In a similar manner, homodyne interferometers have been used to perform vibrometery measurements using relatively simple processing electronics. Also, single beam scanning heterodyne interferometers that use a single detector and electronic signal processor have been used. However, homodyne interferometers suffer from non-linearity effects at large vibration amplitudes, low signal-to-noise ratio caused by laser intensity fluctuations, and inverse frequency (i.e., 1/f) detector noise. Scanning interferometers use a single detector and electronic signal processor, but cannot simultaneously acquire vibration data from all locations on a device under test
U.S. Pat. No. 6,972,846, by Lal et al., issued Dec. 6, 2005 discloses a multi-beam laser Doppler vibrometer that simultaneously measures velocity, displacement, and vibration history of multiple locations on an object. A beam of coherent light is split into an object beam and a reference beam. The object beam is divided into a plurality of object beams to simultaneously illuminate multiple locations on the object under inspection. The reference beam is frequency shifted and split into a corresponding plurality of frequency-shifted reference beams. A portion of each object beam is reflected by the object as a modulated object beam. The plurality of modulated object beams are collected and respectively mixed with the plurality of frequency-shifted reference beams to provide a plurality of beam pairs. Each beam pair may be focused onto a photodetector or an optical fiber connected to a photodetector.
However, U.S. Pat. No. 6,972,846 teaches of a vibrometer implementation that requires a diffractive optical element (DOE) to generate multiple object beams. Since all beams are encoded with the same heterodyne frequency, separate detectors and processing electronics are required for each object beam.
In view of the foregoing, there is a need in the art for systems and methods for efficiently measuring a wavefront of an optical signal. Similarly, there is a need for systems and methods for efficiently measuring object size and movement. Accordingly, there is a need for such systems and methods to functions with a minimal hardware elements. Particularly, there is a need for such systems and methods to measure an optical signal wavefront within minimal number of sensor components. These and other needs are met by the present invention as detailed hereafter.